1. Field of the Invention
The present invention generally relates to a high frequency connector having a common mode choke coil (CMC), and more particularly, to a high frequency connector having a CMC equipped therein for filtering a common mode noise.
2. Description of Related Art
Currently, a typical high frequency Ethernet may achieve a transmission speed up to 1000M bps or 1 Giga bps. Such high frequency signals are very sensitive to electro-magnetic interference (EMI), and cross talks are very likely to occur among a multiple high frequency signals. Unshielded twisted pair (UTP) cables are relative cheap and usually adopted as transmission media for connecting or transmitting a high frequency Ethernet. Such a UTP cable structurally differs from those expansive coaxial cables which are often used for transmitting high frequency signals in that the UTP cable does not have a metal shielding layer. Therefore, when a high frequency signal is transmitted along the UTP cable, the UTP cable is incapable of providing an effective EMI shield and thus the high frequency signal transmitted in the UTP cable is very likely to interfere with other electronic apparatus and cause noises thereby.
However, there is no effective and economic solution provided for completely overcoming the disadvantage of the UTP cable. As such, noises of the UTP cable is usually dealt by signal conditioning. As well known, the high frequency Ethernet transmits differential mode signals with a differential mode. Unfortunately, a common mode signal occurred in the UTP cable often interferes other electronic apparatuses. Such a common mode signal is also known as a common mode noise. Correspondingly, a conventional Ethernet signal conditioning method introduces a common mode choke coil (CMC) in a high frequency connector of an end of the UTP cable. This CMC is adapted to attenuate energy of the common mode signal.
U.S. Pat. No. 7,153,163 discloses a modular jack for Ethernet application, in which a CMC is directly equipped to a high frequency connector. As shown in FIG. 1, there are shown eight signal terminals RJ-1, RJ-2, RJ-3, RJ-4, RJ-5, RJ-6, RJ-7, and RJ-8 of the high frequency connector of U.S. Pat. No. 7,153,163 respectively coupled to four CMCs, CMC1, CMC2, CMC3 and CMC4 for filtering away common mode noises from four sets of differential mode signals transmitted by an input terminal of the high frequency connector.
FIG. 2A is a schematic diagram illustrating an electromagnetic filed applied by a set of differential mode signals on a conventional CMC. Referring to FIG. 2A, an ordinary CMC typically includes a pair of wires L1 and L2 wounded on an iron core F. The pair of wires L1 and L2 is adapted for transmitting a set of common mode signals or differential mode signals. Supposing a set of differential mode signals are transmitted by the wires L1 and L2 via CMC1, a current I1 is defined as being transmitted from a node N11 of the wire L1 to another node N12 of the wire L1 via CMC1, and another current I2 is defined as being transmitted from a node N22 of the wire L2 to another node N21 of the wire L2 via CMC1. In this case, the wires L1 and L2 generate electromagnetic fields Ed1, Ed2 respectively. The electromagnetic fields Ed1, Ed2 are opposite in direction and can be substantially counteracted one by another. In such a way, differential mode signals transmitted by the wires L1 and L2 have almost no variation in resistance when flowing through CMC1. As such, there is almost no attenuation of an energy transmitted from the wire L1 to the wire L2 when flowing through CMC1.
FIG. 2B is a schematic diagram illustrating an electromagnetic filed applied by a set of common mode signals on a conventional CMC. Referring to FIG. 2B, supposing 11 and 12 are a set of common mode signals transmitted by the wires L1 and L2 via CMC1. The current I2 is considered as a being transmitted from the node N21 of the wire L2 to another node N22 of the wire L2 via CMC1, in which electromagnetic fields Ec1, Ec2 generated by the wires L1 and L2 are identical in direction. In such a way, the electromagnetic field generated by the wires L1 and L2 at the iron core F is a sum of the electromagnetic files of Ec1 and Ec2. In such a way, the common mode signals transmitted by the wires L1 and L2 cause apparent variation in resistance when flowing through CMC1. As such, energy of the common mode signals in the wires L1 and L2 is converted into heat and radiation and attenuated thereby due to return loss. Therefore, the energy of the common mode signal drastically attenuates when flowing through CMC1.
In summary, when flowing through the CMC1, a common mode signal causes an energy radiation. Therefore, when such a CMC1 is equipped into a high frequency connector, the CMC1 becomes a radiation source, because there is a set of common mode signals flowing through the CMC1. Accordingly, the differential mode signals flowing through the CMC1 unfortunately suffer EMI. This usually occurs in high frequency connectors having metal shields, in which the radiation of the radiation source is restricted within a range of a shielding housing.